A short update regarding performance and footprint in memory.
The extra code size needed for the library for the scheduler, the task wrapper and a single call to yield in the main loop() is 1700 bytes on AVR This looks cool especially considering that the library is written Object Oriented and use extensively the benefit of virtual methods. 
on Teensy 3 (ARM) we end up surprisingly with 4900 bytes more for the library. this is not really a problem as the chip got 128K program size, but I will investigate why sometime.
RAM memory used by the library variable is not significant here but is mainly impacted by the size of the stacks allocated to tasks. Hopefully this can be sized to the very minimum by using the library method stackLeft to monitor the unused space.
regarding performance, with a recent update in the yield method (to be published wednesday), we end up with a total time of 45us for switching between 2 tasks on a Teensy++2.0 and 55us on Arduino Uno and less than 10us on Teensy 3 (arm).
this time includes timing checking and storage with some calls to (un)famous millis as this library doesnt need any timer or interrupts.
this is anyway good performance which provide a 5% max time lost for scheduling 4 task runing on AVR (3+main loop) and less than 1% on Teensy 3.
unless you do realtime missile cruise calculation you d be safe , right ?
here we go
any body got a DUE to try this fancy lib ?? ! 